1. Field of the Invention
The present invention generally relates to implantable medical devices, such as pacemakers, and, in particular, to techniques for detecting and monitoring mechanical dyssynchronicity of the heart.
2. Description of the Prior Art
Current implantable cardiac resynchronization therapy (CRT) devices are designed to improve congestive heart failure symptoms in cardiomyopathy patients with electromechanical dyssynchrony. In particular, CRT devices are recommended for patients with prolonged QRS suggesting dyssynchronous heart activity. However, up to 30% of the patients carrying CRT devices do in fact not respond to the treatment, so called non-responders. This might depend on the fact that the there is discrepancy in the mechanical dyssynchrony and the electrical dyssynchrony, i.e. there is not a 100% correlation between the mechanical dyssynchrony and the electrical dyssynchrony. Today, in principle, all algorithms for pacing RV and LV in order to reduce or eliminate the dyssynchrony is based on electrical signals, with the exception for echo-cardiography or MR based technologies, which are unsuitable for implantable medical devices. Hence, the information acquired and used for reduction or elimination of the mechanical dyssynchrony is based solely on electrical dyssynchrony.
A large number of studies have shown that intra-cardiac impedance reflects the mechanical activity of the heart and thus provides a good basis for determining a mechanical dyssynchrony of a heart. However, intra-cardiac impedance is a very challenging parameter in that it, for example, varies significantly from patient to patient. The frequency content may for example vary greatly from patient to patient in spite of an identical filtering and signal processing, and the morphologies of the impedance signals may vary significantly. In addition, intra-cardiac impedance is very sensitive to the specific conditions during which the measurements were made such as the posture of the patient, the lead type and the lead position as well as to varying physiological parameters such as heart rate and respiratory rate. In FIG. 1 it is illustrated how measure intra-cardiac impedance may vary from patient to patient. It should be noted that the impedance is measured using the same measurement vector in each patient and for the same patient posture. All the waveforms shown in FIG. 1 are amplitude and time normalized and made up of ˜30 second time averages and data from 19 patients is shown. The waveform indicated by the black, thick line is the ensemble average of all the other waveforms. Accordingly, despite the fact that intra-cardiac impedance is a very promising parameter for characterizing the mechanical activity of a heart there are a number of problems associated with measuring the impedance that have to be solved. The prior art discloses a number of approaches without presenting an adequate solution. For example, in US 2007/0191901 a cardiac resynchronization therapy (CRT) device using intra-cardiac impedance for determining systolic and diastolic cardiac performance is shown. US 2007/0191901 focus on gathering impedance data during, for example, myocardial thickening. In order to obtain a measure of the impedance data gathered during the myocardial thickening, the impedance is integrated from the onset of systole to time of peak contractility. Under conditions of normal or increased contractility this integrated value will have a greater value. Another measure of the cardiac performance discussed in US 2007/0191901 is a relation between the integral of the impedance over systole and the integral of the impedance over diastole. A larger area under the initial portion (during the systolic ejection phase) of the impedance curve will denote better systolic performance and a smaller area under the latter portion of the impedance curve will indicate more optimal lusitropic properties without regional post-systolic myocardial thickening. Thus, an optimized systolic and diastolic function will lead to an increased measure (i.e. the numerator will increase and the denominator will decrease) between the two integrals. The impedance information can be derived by acquiring the impedance between electrode pairs that transverse small myocardial segments. For example, measurements made between bipolar laterally positioned LV leads and the can of the device will reflect data more representative of lateral LV performance and electrodes placed in the interventricular septum or RVOT will generate data that reflects inter-ventricular septal ventricular performance.
Thus, significant efforts have been made within the art to develop devices and methods for detecting and monitoring mechanical dyssynchronicity of a heart. However, even though, as discussed above, intra-cardiac impedance constitutes a good basis for determining a mechanical dyssynchrony of a heart, one of the difficulties with providing a reliable measure of the dyssynchronicity of the heart is to overcome the problems with obtaining stable and reliable impedance data. Furthermore, in order to provide a reliable measure of the dyssynchronicity of the heart, the impedance data must also be processed in a proper and adequate way.
Consequently, there is still a need within art for an improved device and method that are capable of delivering a stable and reliable measure of a mechanical dyssynchrony of a heart of a patient.